Summary This proposal aims to develop and validate MRI methods that exploit novel contrast mechanisms based on chemical exchange and diffusion which can detect changes in the tissue microenvironment, quantify intrinsic micro-structural features, and promise to improve the ability to discriminate pathological processes. These studies will provide imaging biomarkers that can quantitatively describe tissue microstructure and composition in diseases including tumors and stroke, and provide new information for both clinical and pre-clinical applications. At higher field strengths (3T and above) different relaxation mechanisms dominate the behaviors of MR signals compared to lower fields, providing new opportunities to characterize tissues. In particular, spin- lattice relaxation rates (R1?) in the rotating frame using spin-locking sequences become dominated by exchange effects rather than by dipole-dipole interactions, with additional contributions from diffusion through susceptibility gradients that mimic exchange. These processes are affected by very different factors than dipolar interactions, and they can be exploited to provide new types of image contrast. Moreover, R1? varies with the locking field amplitude, and that variation (the R1? dispersion or R1?D) provides a way to derive the parameters of the exchange or to produce new types of parametric images that are sensitive to the changes that occur with pathology. The proposal therefore has three main goals. [1] We have analyzed and measured the effects of diffusion through magnetically inhomogeneous media on R1?D and shown how dephasing effects in the presence of microscopic field gradients from variations in susceptibility produce changes in R1? over a range of low locking field amplitudes. Analysis of this dispersion gives the magnitudes and spatial scales of field gradients, and reveals the dimensions of magnetic inhomogeneities within tissues, such as the micro- vasculature. We will show how such spin lock methods can be used to characterize the dimensions of the micro-vasculature in the normal brain and tumors, and validate the results using quantitative microscopic imaging and other methods. [2] We will also further develop, interpret and apply R1?D imaging at higher locking fields in order to measure proton exchange rates and create new types of parametric image that emphasize the contributions of protons exchanging at specific rates. Moreover, we aim to show these exchange-sensitive images detect early changes in stroke, are affected by pH, and can detect the administration of exogenous ?exchange contrast? agents. We will thus show R1?D imaging provides a novel approach to exploiting exchange-based effects with some advantages compared to techniques such as CEST. [3] Finally, we will demonstrate these methods can be translated to clinical applications and will optimize their implementation, demonstrate their ability to produce novel parametric images based on diffusive and chemical exchange, and assess their reproducibility at 3T. Overall these studies will provide a firm foundation and validation for the use of a new class of exchange-sensitive MR images for clinical and pre-clinical applications.